2.37.1 regression: event.data is null in +layout.ts when +page.ts load throws error()

[noaidi]

Describe the bug

After upgrading from @sveltejs/kit@2.37.0 to 2.37.1, I noticed that if a +page.ts load function throws error(), then in the browser the +layout.ts load function receives event.data as null.

This regression did not occur in 2.37.0 and happens in 2.37.1 and all later versions(2.38.1).

Reproduction

1. Create src/routes/some/path/+page.js:

   import { error } from '@sveltejs/kit';
   
   export async function load() {
     throw error(404, 'Not found');
   }

2. In src/routes/+layout.js:

   export async function load(event) {
     console.log('layout data:', event.data); // This should contain the data set in +layout.server.js.
     return {};
   }

3. Navigate to /some/path in the browser.

Logs

System Info

System:
    OS: macOS 15.6.1
    CPU: (8) arm64 Apple M1
    Memory: 1.84 GB / 16.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 24.7.0 - /opt/homebrew/bin/node
    npm: 11.5.1 - /opt/homebrew/bin/npm
    pnpm: 10.15.1 - /opt/homebrew/bin/pnpm
  Browsers:
    Brave Browser: 140.1.82.161
    Chrome: 140.0.7339.133
    Safari: 18.6
  npmPackages:
    @sveltejs/adapter-vercel: ^5.10.2 => 5.10.2
    @sveltejs/kit: ^2.37.1 => 2.37.1
    @sveltejs/vite-plugin-svelte: ^6.2.0 => 6.2.0
    svelte: ^5.38.10 => 5.38.10
    vite: ^7.1.5 => 7.1.5

Severity

blocking an upgrade

Additional Information

No response

[sillvva]

Duplicate of #14387

Fixed by #14395